The present invention relates to salt water resisting machines or machine parts made of austenitic cast iron having resistance to stress corrosion cracking in salt water which contains chloride ion (Cl.sup.-) such as natural seawater, concentrated seawater or diluted seawater.
Austenitic cast iron, i.e., ASTM A-436 of the flaky graphite type or ASTM A-439 of the nodular graphite type, containing 13.5-22 wt % or 28-37 wt % of Ni (all percents noted hereinafter are by weight) exhibits good corrosion resistance or good heat resistance and is preferentially used in machines or machine parts intended for use under corrosive environments associated with the handling of salt water and the like, or under high temperature environments.
While various species of austenitic cast iron are known, austenitic cast iron containing 13.5-22 wt % of Ni, i.e., ASTM A-436 Type 1, Type 1b, Type 2, Type 2b, ASTM A-439 Type D-2 or Type D-2B, is used in machines or machine parts intended for use in salt water, and austenitic cast iron containing more than 28% Ni is used in equipment at chemical plants which is required to have high heat resisting properties. Austenitic cast iron with a nickel content of 22% or below provides sufficient corrosion resistance for machines or machine parts intended for use in salt water. Because of this fact and the economical advantage resulting from low nickel content, in no case has austenitic cast iron with a nickel content of 28% or higher been used as a material for machines or machine parts intended for use in salt water.
Austenitic cast iron species are available that contain up to 24% of nickel and have an increased Mn content, and Type D-2C is an example of such species. However, they are exclusively used as materials for machines or machine parts intended for use at cryogenic temperatures, and in no case have they been used in corrosion-resistant machines or machine parts intended for use in salt water.
The resistance of austenitic cast iron to general corrosion is such that the corrosion rate is only about 0.1 mm/year in seawater at ordinary temperatures. Unlike mild steels and cast iron, the increase in the rate of general corrosion in austenitic cast iron situated in flowing seawater over that in standing seawater is negligible, and if the seawater flows faster, the rate of corrosion is even seen to decrease. Additionally, austenitic cast iron is not susceptible to localized corrosion such as crevice corrosion and pitting corrosion that are common to stainless steel. Because of the balanced resistance to various forms of corrosion, austenitic cast iron is extensively used in machines and machine parts that handle seawater and other corrosive fluids.
Cases, however, have been reported to the assignee of the present invention of machines or machine parts made of austenitic cast iron handling natural seawater or concentrated seawater developing cracks a considerable time after the start of service.